1. Field of the Invention
The present invention generally relates to methods for embedding micropores in microchannels and devices thereof using a single-level etching process and to tailoring the physical characteristics of these micropores for biological and chemical analysis.
2. Description of the Related Art
The concept of micro total analysis systems (microTAS) introduced for chemicals in the early 1990s by Manz et al. has been embraced by researchers in the emerging field of systems biology for studying the intra- and inter-cellular workings of a cell. See Manz et al. (1990) Sensors and Actuators B-Chemical 1:244-248, and Breslauer et al. (2006) Molecular Biosystems 2:97-112. These microfluidic platforms enable multiplexed studies at the single-cell level in a controlled microenvironment with the inherent advantages of fast reaction times, small reagent consumption, and parallelization.
The vast majority of these devices are focused on a single functionality or one basic operation. See El-Ali et al. (2006) Nature 442:403-411. Only a handful of truly integrated cell-based microfluidic platforms with multiple components have been reported. Examples include a high-density array with hundreds of individually addressable cell chambers; a device for single-cell manipulation, lysis, amino acid/protein labeling, and separation; and a microfluidic chip to continuously monitor secreted insulin from multiple independent islets of Langerhans. See Thorsen et al. (2002) Science 298:580-584; Wu et al. (2004) PNAS USA 101:12809-12813; Huang et al. (2007) Science 315:81-84; and Dishinger & Kennedy (2007) Anal. Chem. 79:947-954. The majority of these multiplexed platforms are fabricated from poly(dimethyl-siloxane) (PDMS) because microstructures such as valves, weirs, and micropores can be easily embedded within a network of microfluidic channels. See Duffy (1998) Anal Chem. 70:4974-4984; and Unger et al. (2000) Science 288:113-116; and Di Carlo et al. (2006) Anal. Chem. 78:4925-4930; and Seo et al. (2004) Applied Physics Letters 84:1973-1975.
Historically, silicon and glass have been the preferred substrate for the fabrication of microfluidic chips. See McCreedy (2000) Trac-Trends in Analytical Chemistry 19:396-401; and Ziaie et al. (2004) Advanced Drug Delivery Reviews, 56:145-172. Despite the wide academic acceptance of PDMS-based microfluidic chips, glass remains an attractive alternative for many biological applications because of its large optical transition range (180-2500 nm); high resistance to mechanical stress, heat, and chemicals; high electric isolation; absence of porosity; and high biocompatibility through its well-studied surface chemistry.
Of the three major glass etching techniques—mechanical, dry, and wet—the most common microfabrication method practiced is isotropic wet etching. Well characterized in the literature, this straight-forward fabrication method uses a photolithography mask to define features on the surface of the wafer. See Jacobson et al. (1995) Anal. Chem. 67:2059-2063; Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton, Fla., 1997. Timed exposure to chemical etchants such as HF dissolves the Si—O—Si bonds in the glass isotropically, generating a D-shaped channel with a smooth surface and a constant depth. However, this technique is limited in that shallow structures (i.e. channels and weirs) are difficult to manufacture within larger channels without performing a multi-level wet etch or a combination of dry and wet etch—a costly and time-consuming operation because of the multiple masks and alignment steps needed between the different levels.
Therefore, a need exists for methods for making microstructures (e.g. micropores, ridges, etc.) within microfluidic channels for on-chip cell manipulation using single-step isotropic wet etch.